TNA's downfall would come if Vince McMahon raided all of the TNA Originals, plus some of the old WWE guys (Kurt Angle, specifically)
I dont know if they would even make a DVD of TNA, or if they did, it wouldnt be like the "Fall of WCW" one. WCW was actual competition, which is not TNA at all.
Personally, as much as I like TNA, I'd like to see Samoa Joe / AJ Styles / Beer Money / Christopher Daniels on the WWE roster, just as long as they bring the X-Division title with them.